Oil-extended rubber, process for producing the same, rubber composition, and crosslinked object

ABSTRACT

An oil-extended rubber comprising 100 weight parts of a rubber ingredient comprising a conjugated diene rubber wherein the ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) is in the range of 1.2 to 2.2, and the conjugated diene monomer units have a vinyl bond unit content of at least 20%, and 5-100 weight parts of a process oil having a total acid value of not larger than 1.0 mgKOH/g, a pour point of not higher than 50° C. and an aromatic carbon content of at least 20%. A rubber composition comprising 100 weight parts of the above-mentioned rubber ingredients, 5-100 weight parts of the process oil and 10-200 weight parts of a reinforcing agent. This rubber composition gives a crosslinked article having high tensile strength, high abrasion resistance and reduced heat build-up, and is useful, for example, as an automobile tire material.

TECHNICAL FIELD

This invention relates to an oil-extended rubber used as a material fora crosslinked rubber article having excellent tensile strength, abrasionresistance and reduced heat build-up; a process for producing theoil-extended rubber; a rubber composition comprising the oil-extendedrubber and a reinforcing agent; and a crosslinked rubber article made bycrosslinking the rubber composition.

BACKGROUND ART

An attempt has hitherto been made for improving tensile strength andabrasion resistance of rubber. Widely adopted examples of the attemptinclude a method of using a rubber having a high molecular weight and amethod of modifying a rubber by incorporating a reinforcing agenttherein. However, the high molecular weight rubber has a high viscosity,and the rubber composition having incorporated therein a reinforcingagent is rigid. Therefore, these rubbers have poor processability.

Serious consideration is now given for material resource saving andenvironmental protection, and thus a severe requirement is imposed forreducing fuel consumption of automobiles. As for automobile tires, forwhich a rubber material is enormously consumed, there is an increasingdemand for providing tires having a reduced rolling resistance to reducefuel consumption of automobiles.

A crosslinked rubber product exhibiting a low heat build-up is generallyused for producing tires having a reduced rolling resistance. Acrosslinked rubber product made from a diene rubber composition havingincorporated therein silica as a reinforcing agent has been proposed asa crosslinked rubber exhibiting a low heat build-up. However, thecrosslinked rubber product made from a silica-incorporated diene rubbercomposition has a problem such that abrasion resistance and tensilestrength are poor as compared with a crosslinked rubber product madefrom a widely used diene rubber composition having carbon blackincorporated therein.

To enhance the abrasion resistance of a crosslinked rubber product madefrom a silica-incorporated rubber composition, a method of increasingthe molecular weight of rubber and a method of increasing the amount ofsilica to be incorporated in a rubber composition are adopted. Thesemethods still have problems such that processability of the rubbercomposition is reduced and a crosslinked rubber product made therefrombecomes rigid with the result of reduction of road surface holding oftires.

To impart good processability to a rubber composition, a method of usinga rubber having a specific molecular weight distribution was proposed inJapanese Unexamined Patent Publication No. S61-255908. However, acrosslinked rubber product made from this rubber composition has aproblem in that heat build-up is large.

To improve processability of a rubber composition, a method ofincorporating a process oil therein was proposed, for example, inJapanese Unexamined Patent Publication No. H7-292161. However, acrosslinked rubber product made from the oil-extended rubber compositionhas greatly reduced abrasion resistance and tensile strength.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an oil-extended rubberwhich gives a rubber composition exhibiting good processability when areinforcing agent is incorporated therewith, and which rubbercomposition gives a crosslinked rubber article having high tensilestrength, high abrasion resistance and low heat build-up.

The present inventors carried out research into a rubber materialexhibiting good processability even when a reinforcing agent isincorporated therewith, and giving a crosslinked rubber product havinghigh tensile strength, high abrasion resistance and low heat build-up.As a result thereof, it has been found that the above object can beachieved by an oil-extended rubber comprising a specific rubberingredient extended with a specific process oil. Thus, the presentinvention has been completed.

In one aspect of the present invention, there is provided anoil-extended rubber comprising 100 parts by weight of a rubberingredient comprising a conjugated diene rubber wherein the ratio(Mw/Mn) of weight average molecular weight (Mw) to number averagemolecular weight (Mn), as measured by gel permeation chromatography andexpressed in terms of those of polystyrene, is in the range of 1.2 to2.2, and the conjugated diene monomer units have a vinyl bond unitcontent of at least 20%, and 5 to 100 parts by weight of a process oilhaving a total acid value of not larger than 1.0 mgKOH/g, a pour pointof not higher than 50° C. and an aromatic carbon content of at least 20%as measured by the Kurtz analysis method.

In another aspect of the present invention, there is provided a processfor producing the above-mentioned oil-extended rubber, which comprisesthe steps of (1) polymerizing a conjugated diene monomer or monomersalone, or a combination of a conjugated diene monomer or monomers with amonomer copolymerizable therewith by using an organic active metalinitiator in the presence of a polar compound in a hydrocarbon medium;(2) adding a terminator to a polymerization system to terminate thepolymerization; (3) adding 5 to 100 parts by weight, based on 100 partsby weight of the total monomers, of a process oil having a total acidvalue of not larger than 1.0 mgKOH/g, a pour point of not higher than50° C. and an aromatic carbon content of at least 20% as measured by theKurtz analysis method; and (4) removing the hydrocarbon medium andrecovering the thus-produced oil-extended rubber.

In a further aspect of the present invention, there is provided a rubbercomposition comprising (i) 100 parts by weight of a rubber ingredientcomprising a conjugated diene rubber wherein the ratio (Mw/Mn) of weightaverage molecular weight (Mw) to number average molecular weight (Mn),as measured by gel permeation chromatography and expressed in terms ofthose of polystyrene, is in the range of 1.2 to 2.2, and the conjugateddiene monomer units have a vinyl bond unit content of at least 20%, (ii)5 to 100 parts by weight of a process oil having a total acid value ofnot larger than 1.0 mgKOH/g, a pour point of not higher than 50° C. andan aromatic carbon content of at least 20% as measured by the Kurtanalysis method, and (iii) 10 to 200 parts by weight of a reinforcingagent.

BEST MODE FOR CARRYING OUT THE INVENTION Oil-Extended Rubber

The oil-extended rubber of the present invention comprises a specificrubber ingredient and a specific process oil.

Rubber Ingredient

A rubber ingredient contained in the oil-extended rubber of the presentinvention comprises as an indispensable ingredient a conjugated dienerubber wherein the molecular weight distribution (Mw/Mn) is in the rangeof 1.2 to 2.2. By the term “conjugated diene rubber” used herein, wemean a rubber comprised of conjugated diene monomer units in thebackbone chain. This polymer includes a homopolymer of a conjugateddiene monomer and a copolymer of at least two conjugated diene monomers,and a copolymer of a conjugated diene monomer and a monomercopolymerizable therewith.

The rubber ingredient may comprise either a single kind of conjugateddiene rubber or a mixture of two or more kinds of conjugated dienerubbers. The rubber ingredient may contain a rubber having no conjugateddiene monomer units. The content of the conjugated diene rubber in therubber ingredient is preferably at least 50% by weight, more preferablyat least 60% and especially preferably at least 70% by weight.

As specific examples of the conjugated diene monomer, there can bementioned 1,3-butadiene, 2-methyl-1,3-butadiene,2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene and 1,3-pentadiene.Of these, 1,3-butadiene and 2-methyl-1,3-butadiene are preferable,1,3-Butadiene is most preferable. These conjugated diene monomers may beused either alone or as a combination of at least two thereof.

The monomer copolymerizable with the conjugated diene monomer is notparticularly limited, and includes, for example, an aminogroup-containing vinyl monomer, a pyridyl group-containing vinylmonomer, a hydroxyl group-containing vinyl monomer, an alkoxylgroup-containing vinyl monomer and an aromatic vinyl monomer.

Of these, an aromatic vinyl monomer is preferable. As specific examplesof the aromatic vinyl monomer, there can be mentioned styrene,α-methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene,2,4-diisopropylstyrene, 2,4-dimethylstyrene, 4-t-butylstyrene,5-t-butyl-2-methylstyrene, monochlorostyrene, dichlorostyrene andmonofluorostyrene. Of these, styrene is especially preferable. Thecopolymerizable monomers may be used either alone or as a combination ofat least two thereof. The kind and amount of the copolymerizable monomeris appropriately chosen depending upon the particular use of rubber.

The content of conjugated diene monomer units in the conjugated dienerubber is such that the lower limit thereof is preferably 50%, morepreferably 60% and especially preferably 70%, and the upper limitthereof is 100%, preferably 95%, more preferably 92% and especiallypreferably 90%. When the content of conjugated diene monomer units istoo small, the properties desired for a conjugated diene rubber tend tobe deteriorated.

In the case where the conjugated diene rubber is a copolymer of aconjugated diene monomer with a copolymerizable monomer, the sequencedistribution of the copolymerizable monomer units in the copolymer isnot particularly limited. However, the content of an isolated shortchain of a copolymerizable monomer unit, i.e., the content of acopolymerizable monomer unit, which is adjacent only to conjugated dienemonomer units in the copolymer, is such that the lower limit thereof ispreferably 50% by weight, more preferably 60% and especially preferably70% by weight, and the upper limit thereof is 100% by weight, based onthe total content of the copolymerizable monomer units. The content of along sequence of copolymerizable monomer units, i.e, a sequence composedof at least eight sequential copolymerizable monomer units, in thecopolymer, is such that the lower limit thereof is 0% by weight, and theupper limit thereof is preferably 2% by weight, more preferably 1% byweight and especially preferably 0.5% by weight, based on the totalcontent of the copolymerizable monomer units. When the sequencedistribution of the copolymerizable monomer units is satisfactory asmentioned above, a crosslinked rubber product made from a rubbercomposition comprising the oil-extended rubber has excellent tensilestrength, abrasion resistance and heat build-up.

The content of vinyl bond units (i.e., the sum of 1,2-vinyl bond unitsand 3,4-vinyl bond units) in the conjugated diene rubber constitutingthe rubber ingredient is such that the lower limit thereof is 20% byweight, preferably 40% by weight and more preferably 50% by weight, andthe upper limit thereof is preferably 90% by weight, more preferably 85%by weight and especially preferably 80% by weight, based on the totalweight of the conjugated diene monomer units. If the content of vinylbond units is too small, a crosslinked rubber product made from therubber composition tends to have poor tensile strength and poor abrasionresistance. In contrast, a conjugated diene rubber having a too largecontent of vinyl bond units is difficult to produce. Conjugated dienemonomer units other than vinyl bond units are 1,4-bond units, and the1,4-bond units in a conjugated diene rubber may be either 1,4-cis-bondunits or 1,4-trans-bond units, or both of 1,4-cis-bond units and1,4-trans-bond units.

The weight average molecular weight (Mw) of the conjugated diene rubberas measured by gel permeation chromatography (GPC) and expressed interms of that of polystyrene is such that the lower limit thereof ispreferably 200,000, more preferably 400,000 and especially preferably600,000, and the upper limit thereof is preferably 2,000,000, morepreferably 1,500,000 and especially preferably 1,200,000. When theweight average molecular weight (Mw) of the conjugated diene rubber istoo small, a crosslinked rubber product made from the rubber compositiontends to have large heat build-up, poor abrasion resistance and poortensile stress. In contrast, when Mw is too large, the rubbercomposition tends to have poor processability. The conjugated dienerubber may be such that it has been modified with a coupling agent toenhance the molecular weight.

The molecular weight distribution of the conjugated diene rubber,namely, the ratio (Mw/Mn) of weight average molecular weight (Mw) tonumber average molecular weight (Mn) is such that the lower limitthereof is 1.2, preferably 1.3 and more preferably 1.4, and the upperlimit thereof is 2.2, preferably 2.1 and more preferably 2.0. If themolecular weight distribution (Mw/Mn) of the conjugated diene rubber istoo small, the rubber composition tends to have poor processability. Incontrast, if Mw/Mn is too large, a crosslinked rubber product made fromthe rubber composition tends to have large heat build-up, poor tensilestrength and poor abrasion resistance.

The rubber ingredient preferably contains a conjugated diene rubberhaving a polar group, especially a conjugated diene rubber having anamino group or an alkoxysilyl group. In the case where the rubberingredient contains a conjugated diene rubber having a polar group, auniform dispersion of a reinforcing agent in an oil-extended rubber canbe easily obtained at a step of preparing a reinforcing agent-containingrubber composition, and, if silica is used as a reinforcing agent, ahigh adhesion between the oil-extended rubber and silica can beobtained. The content of the conjugated diene rubber having a polargroup in the rubber ingredient is such that the lower limit thereof ispreferably 10% by weight, more preferably 20% by weight, especiallypreferably 30% by weight and most preferably 40% by weight, and theupper limit thereof is 100% by weight. The conjugated diene rubberhaving a polar group is prepared by copolymerization of a monomer havinga polar group, or modification of a conjugated diene rubber with a polargroup-containing compound.

Process Oil

The process oil used for the preparation of an oil-extended rubber ofthe present invention has a small total acid value. The upper limit ofthe total acid value is 1.0 mgKOH/g, preferably 0.4 mgKOH/g and morepreferably 0.1 mgKOH/g. The total acid value means an amount of thetotal acid ingredients in the process oil, as expressed by an amount ofpotassium hydroxide required for neutralize the total acid ingredientscontained in 1 gram of the process oil. If a process oil having a toolarge total acid value is used, a crosslinked rubber product made fromthe rubber composition tends to have large heat build-up and poorabrasion resistance.

The process oil can be such that is prepared by allowing a process oilhaving a total acid value of larger than 1.0 mgKOH/g to react with abasic compound to reduce the total acid value to 1.0 mgKOH/g or smaller.It is possible that a process oil having a total acid value of largerthan 1.0 mgKOH/g is incorporated with a basic compound in an amountrequired for reducing the total acid value to 1.0 mgKOH/g or smaller,and a conjugated diene rubber is extended with the thus-obtained processoil/basic compound mixture whereby the total acid value of larger than1.0 mgKOH/g is reduced to 1.0 mgKOH/g or smaller during extending.

The process oil used in the present invention has a pour point of nothigher than 50° C., preferably not higher than 30° C. and morepreferably not higher than 10° C. The lower limit of the pour point isnot particularly limited, but is preferably −20° C. and more preferably−10° C. When the pour point is not higher than 50° C., a crosslinkedproduct made from the rubber composition exhibits excellent heatbuild-up. A process oil having a too high pour point exhibits a highviscosity and a conjugated diene rubber is occasionally difficult toextend with the process oil. In contrast, a process oil having a too lowpour point tends to give a crosslinked rubber product having poorabrasion resistance.

The process oil used in the present invention has an aromatic carboncontent (CA%) of at least 20%, preferably at least 22% and morepreferably at least 25% as measured by the Kurtz analysis method. Theupper limit of the aromatic carbon content is not particularly limitedand is 100%. The process oil preferably has a paraffinic carbon content(CP%) of not larger than 55%, more preferably not larger than 50% andespecially preferably not larger than 45%. The lower limit of theparaffinic carbon content is not particularly limited. If CA% is toosmall or CP% is too large, a crosslinked rubber product made from therubber composition tends to have poor tensile strength and poor abrasionresistance.

The content of an aromatic polyoyclic hydrocarbon in the process oilused in the present invention is preferably smaller than 3%. The contentof an aromatic polycyclic hydrocarbon in the process oil can be measuredby the method of IP346 (testing method according to the InstitutePetroleum of United Kingdom).

Oil-Extended Rubber

The oil-extended rubber of the present invention comprises theabove-mentioned conjugated diene rubber and the above-mentioned processoil. The amount of the process oil is 5 to 100 parts by weight based on100 parts by weight of the rubber ingredient. The lower limit of theprocess oil is preferably 10 parts by weight, more preferably 20 partsby weight, and the upper limit thereof is preferably 80 parts by weight,more preferably 60 parts by weight.

The Mooney viscosity of the oil-extended rubber is not particularlylimited, but the lower limit thereof is preferably 20, more preferably25 and especially preferably 30, and the upper limit thereof ispreferably 100, more preferably 90 and especially preferably 80. If theMooney viscosity is too small, a crosslinked rubber product made fromthe rubber composition is liable to have large heat build-up and poorabrasion resistance. In contrast, if the Mooney viscosity is too large,the rubber composition is liable to have poor processability.

Process for Producing Oil-Extended Rubber

The above-mentioned oil-extended rubber is produced by a processcomprising (1) a step of polymerizing a conjugated diene monomer ormonomers, (2) a step of terminating polymerization, (3) a step ofincorporating the process oil, and (4) a step of recovering theoil-extended rubber. The process for producing the oil-extended oil willbe explained in the sequence of steps.

Polymerization Step

A conjugated diene monomer or monomers are polymerized by using anorganic active metal initiator in the presence of a polar compound in ahydrocarbon medium. The conjugated diene monomer or monomers are usedeither alone or as a combination thereof with a monomer copolymerizabletherewith. The kind of the conjugated diene monomer or monomers, thekind of the optional copolymerizable monomer, and the ratio ofcomonomers are described above with reference to the conjugated dienerubber.

The polymerization is carried out in a hydrocarbon medium. Thehydrocarbon solvent used in the polymerization step includes aliphatichydrocarbons, alicyclic hydrocarbons and aromatic hydrocarbons. Thehydrocarbon may be used either alone or as a combination of at least twothereof. The amount of hydrocarbon is preferably such that theconcentration of monomers is in the range of 1% to 30% by weight.

An organic active metal initiator is used as a polymerization initiator.The organic active metal initiator preferably includesorgano-alkali-metal compounds. As specific examples of theorgano-alkali-metal compound, there can be mentioned organolithiumcompounds such as an organo-monolithium compound and a polyvalentorganolithium compound, organosodium compounds, and organopotassiumcompounds. Of these, organolithium compounds are preferable.oragano-monolithium compound is especially preferable. The organicactive metal initiator may be used alone or as a combination of at leasttwo thereof. The amount of organic active metal initiator isappropriately chosen depending upon the molecular weight of intendedpolymer to be produced, but is preferably in the range of 0.1 to 30milli-mole per 100 g of monomer.

To obtain a conjugated diene rubber having the desired content of vinylbond units, the polymerization is carried out in the presence of a polarcompound. The amount of polar compound is preferably in the range of 0.5to 100 moles per mole of organic active metal initiator. If the amountof polar compound is too small, the content of vinyl bond units in aconjugated diene polymer is reduced. As specific examples of the polarcompound, there can be mentioned ethers such as tetrahydrofuran, diethylether, dioxane, ethylene glycol dimethyl ether, ethylene glycol dibutylether, diethylene glycol dimethyl ether and diethylene glycol dibutylether; tertiary amines such as tetramethylethylenediamine,trimethylamine, triethylamine, pyridine and quinuclidine; alakli metalalkoxide compounds such as potassium-t-amyloxide andpotassium-t-butoxide; and phosphine compounds such astriphenylphosphine. Of these, ethers and tertiary amines are preferable.

The polymerization reaction is carried out preferably at a temperatureof −78 to 150° C. The polymerization manner can be either batchwise orcontinuous. Batchwise polymerization is preferable.

Modification Step

When the polymerization is carried out by using an organic active metalinitiator in the above-mentioned manner, a polymer having an organicactive metal bound to a terminal of the polymer chain thereof isproduced. The thus-produced polymer having an organic active metal boundto a terminal of the polymer chain can be modified by allowing amodifier to react with the polymer. More specifically, when a terminalmodifier is allowed to react with the polymer chain, an atomic groupsuch as a polar group can be introduced to the site of the polymerchain, to which an organic active metal has been bound. The conjugateddiene rubber used in the present invention may be either modified orunmodified. When the conjugated diene rubber is modified, themodification is preferably conducted at a modification step interveningbetween the polymerization step and the polymerization terminating step.

The modifier used is not particularly limited, but a terminal modifieror a coupling agent is ordinarily used. When a terminal modifier isplaced in contact with the polymer having an organic active metal boundto a terminal of the polymer chain, a terminal-modified conjugated dienerubber having a polar group introduced to the terminal thereof isproduced. When a coupling agent is placed in contact with the polymerhaving an organic active metal bound to a terminal thereof, a conjugateddiene rubber comprised of a coupled polymer is produced, which is formedby two or more of the organic active metal-bound polymer are boundthrough the coupling agent. As the coupling agent, a coupling agenthaving at least three sites to which the polymer chains are capable ofbeing bound is preferably used.

The terminal modifier used is not particularly limited provided that itis capable of introducing a polar group to a polymer terminal. Among theterminal modifiers, those which are capable of introducing a tertiaryamino group is preferable. As specific examples of such terminalmodifiers, there can be mentioned N,N-di-substituted aminoalkyl acrylateand N,N-di-substituted aminoalkyl methacrylate; acrylamides andmethacrylamides; N,N-di-substituted amino aromatic vinyl compounds;pyridyl group-containing vinyl compounds, N-substituted cyclic amides;N-substituted cyclic ureas, N-substituted aminoketones; N-substitutedaminoaldehydes; N-substituted carbodiimides; and Schiff bases. Theseterminal modifiers may be used either alone or as a combination of atlast two thereof. The amount of terminal modifier is appropriatelychosen depending upon the particular properties required for aconjugated diene rubber, and is preferably in the range of 0.1 to 50equivalent to the organic active metal.

After the terminal modification, a further modification treatment can beconducted. For example, in the case where a tertiary amino group isintroduced to a polymer terminal by terminal modification, the resultingpolymer can be further treated with a quaternarizing agent whereby thetertiary amino group in the polymer is converted to a quaternary aminogroup. As examples of the quaternarizing agent, there can be mentionedalkyl nitrates, potassium alkylsulfates, dialkyl sulfates, alkylarylsulfonates, alkyl halides and metal halides.

The terminal modification is effected by placing a polymer having anorganic active metal bound to a terminal thereof in contact with theterminal modifier. The reaction temperature and reaction time can bechosen in a broad range, but, the temperature and time are preferably inthe ranges of 15 to 120° C. and 1 second to 10 hours, respectively. Themodification percentage is preferably in the range of 10% to 100%. Themodification percentage can be determined based on a calibration curvepreviously prepared, from a ratio (UV/RI) of an absorption intensity(UV) as measured by a visible spectrophotometer to an absorptionintensity (RI) as measured by a differential refractometer in gelpermeation chromatography.

When a coupling agent is placed in contact with a polymer chain havingan organic active metal bound to a terminal thereof, a plurality of thepolymers can be coupled together through the coupling agent at therespective sites of polymers to which the organic active metal has beenbound. The coupling agent is not particularly limited provided that itis capable of giving a coupled polymer, and it includes, for example,tin-containing coupling agents, silicon-containing coupling agents,unsaturated nitrile coupling agent, ester coupling agents, halidecoupling agents and phosphorus-containing coupling agents. Thesecoupling agents may be used either alone or as a mixture of at least twothereof. The amount of coupling agent is preferably in the range of 0.1to 10 equivalent to the organic active metal.

The temperature and time for the coupling reaction are preferably in theranges of 0 to 150° C. and 0.5 second to 20 hours, respectively. Themodification percentage can be appropriately chosen and is preferably inthe range of 10% to 100%. The coupling percentage can be determined frompeaks obtained by a differential refractometer on the GPC measurementconducted before and after the coupling. Namely, an area (A) of a peakobtained after the coupling, which occurs at the same position as thatof a peak obtained before the coupling, is calculated. An area (B) of apeak occurring in a position corresponding to a molecular weight higherthan that of the peak for (A) is calculated. The coupling percentage isa ratio of (B)/[(A)+(B)] in percents.

Polymerization terminating Step

To deactivate an organic active metal bound to a terminal of polymerchain, followed by removal of the organic active metal after completionof the polymerization step or after completion of the modification step,a terminator is added to a polymerization system to terminatepolymerization. The terminator includes, for example, alcohols such asmethanol and isopropanol. The amount of terminator used is preferably inthe range of 0.1 to 50 equivalent to the organic active metal.

Process Oil Incorporating Step

The process oil used in the present invention is incorporated in apolymerization system wherein polymerization has been stopped by theaddition of a terminator, and the mixture is thoroughly stirred touniformly disperse the process oil in the polymerization system.According to the need, additive ingredients can also be incorporated atthis process oil-incorporating step. For example, in the case where thepolymer is heated in the succeeding steps of removing a polymerizationmedium and drying the polymer, a phenolic antioxidant, aphosphorus-containing antioxidant or a sulfur-containing antioxidant ispreferably incorporated at the process oil-incorporating step. Theamount of the antioxidant can be appropriately chosen depending upon thekind of antioxidant.

Recovering Step

In a recovering step, after incorporation of the process oil and otheroptional ingredients, a polymer, i.e., an oil-extended rubber comprisinga polymer and the process oil, is recovered from a polymerizationsystem.

The procedure by which the oil-extended rubber is recovered is notparticularly limited. As examples of the recovering procedure, there canbe mentioned a direct drying procedure wherein a polymerization mixtureis dried to remove a polymerization medium by heating; a procedurewherein a polymerization mixture is introduced in a poor solvent for theobtained rubber to precipitate an oil-extended rubber, and the rubber isrecovered by, for example, filtration, followed by drying to remove thesolvent; and a steam stripping procedure wherein high-temperature steamis blown into a polymerization mixture to remove a polymerization mediumand simultaneously precipitate an oil-extended rubber of a crumb statein an aqueous medium formed from a condensed steam, and the rubber isrecovered by, for example, filtration, followed by drying to remove thewater. If impurities such as metal residue cannot be thoroughly removedin these recovering procedures, a procedure of dissolving theoil-extended oil in a good solvent and then a poor solvent is added toprecipitate a rubber can be repeated, followed by washing and drying.

When the steam stripping procedure is carried out, a dispersing agent ora coagulating aid is incorporated in a polymerization mixture before thesteam stripping, or a dispersing agent or a coagulating aid isincorporated in water in a stripping zone, and the water is blowntogether with high-temperature steam into a polymerization mixture. Asthe dispersing agent, an anionic surface active agent, a cationicsurface active agent and a nonionic surface active agent are generallyused. The amount of the dispersing agent incorporated in water in astripping zone is preferably in the range of 0.1 to 3,000 ppm. As thecoagulating aid, there can be mentioned, for example, water-solublesalts of metal such as lithium, sodium, potassium, magnesium, calcium,aluminum and tin.

To obtain a crumb of oil-extended rubber having a desired particlediameter, the concentration of oil-extended rubber crumb dispersed inwater is preferably in the range of 0.1 to 20% by weight based on theweight of water in a stripping zone. The obtained wet crumb ofoil-extended rubber is preferably dehydrated to a moisture content of 1to 30% by weight. More specifically, the wet crumb is first dehydratedto a moisture content of 35 to 60% by weight by using a dehydrator suchas a rotary screen, a vibrating screen or a centrifugal dehydrator.Then, the wet crumb is further dehydrated to a moisture content of 35 to60% by weight, for example, by a compression squeezing machine such as aroll, a Banbury dehydrator or a screw-extruding type dehydrator. Afterdehydration, the crumb is dried preferably by a dryer such as a screwextruder, a kneader-type dryer, an expander dryer or hot air dryer togive an oil-extended rubber having a moisture content of below 1% byweight.

Rubber Composition

The rubber composition of the present invention comprises 100 parts byweight of a rubber ingredient comprising a conjugated diene rubber, 5 to100 parts by weight of a process oil, 10 to 200 parts by weight of areinforcing agent, and optional ingredients according to the need. Theconjugated diene rubber and the process oil are as explained above withregard to the oil-extended rubber of the present invention. The rubberingredient comprising the conjugated diene rubber and the process oilcan be used in the form of the above-mentioned oil-extended rubber forthe preparation of the rubber composition.

Reinforcing Agent

The reinforcing agent used is not particularly limited, but silica andcarbon black can be used.

The silica is not particularly limited, and includes, for example,dry-process white carbon, wet-process white carbon, colloidal silica,and precipitated silica described in Japanese Unexamined PatentPublication No. S62-62838. Of these, wet-process silica predominantlycomprised of hydrous silicic acid is especially preferable. The silicamay be used either alone or as a combination of at least two thereof.

The specific surface area of silica is not particularly limited, but issuch that the lower limit thereof is preferably 50 m² /g, morepreferably 100 m²/g and especially preferably 120 m² /g, and the upperlimit thereof is preferably 400 m²/g, more preferably 220 m²/g andespecially preferably 190 m²/g, as the nitrogen absorption specificsurface area as measured by the BET method. When the specific surfacearea is within this range, enhancement of tensile strength and abrasionresistance of a crosslinked rubber product made from the rubbercomposition, and reduction of heat build-up thereof can be wellattained. The nitrogen absorption specific surface area is measured bythe BET method according to ASTM D3037-81.

The carbon black is not particularly limited, and includes, for example,furnace black, acetylene black, thermal black, channel black andgraphite. Of these, furnace black is especially preferable, whichincludes various grades such as SAF, ISAF, ISAP-HS, ISAF-LS, IISAF-HS,HAF, HAF-HS, HAF-LS and FEF. The carbon black may be used either aloneor as a combination of at least two thereof.

The specific surface area of the carbon black is not particularlylimited, but is such that the lower limit thereof is preferably 5 m²/g,more preferably 50 m²/g and especially preferably 80 m²/g, and the upperlimit thereof is preferably 200 m²/g, more preferably 150 m²/g andespecially preferably 130 m²/g, as the nitrogen absorption specificsurface area as measured by the BET method. When the specific surfacearea is within this range, tensile strength and abrasion resistance of acrosslinked rubber product made from the rubber composition can begreatly enhanced.

The absorption of dibutyl phthalate (DBP) by the carbon black is alsonot particularly limited, but is such that the lower limit thereof ispreferably 5 ml/100 g, more preferably 50 ml/100 g and especiallypreferably 80 ml/100 g, and the upper limit thereof is preferably 300ml/100 g, more preferably 200 ml/100 g and especially preferably 160ml/100 g. When the absorption of DBP is within this range, tensilestrength and abrasion resistance of a crosslinked rubber product madefrom the rubber composition can be greatly enhanced.

The abrasion resistance can be more conspicuously improved by usinghigh-structure carbon black having a specific surface area of 110 to 170m²/g, as measured by the method of absorption of cetyltrimethylammoniumbromide (CTAB) as described in Japanese Unexamined Patent PublicationNo. H5-230290, and a DBP absorption of 110 to 130 ml/100 g as measuredafter 4 times' repeated compression at a pressure of 24,000 psi (24M4DBP).

The amount of reinforcing agent is such that the lower limit thereof ispreferably 10 parts by weight, more preferably 20 parts by weight andespecially preferably 30 parts by weight, and the upper limit thereof ispreferably 200 parts by weight, more preferably 150 parts by weight andespecially preferably 120 parts by weight, based on 100 parts by weightof the rubber ingredient.

To attain the object of the present invention to a high degree, thereinforcing agent is comprised of either silica alone or a combinationof silica with carbon black. In the case where silica is used incombination with carbon black, the mixing ratio of silica to carbonblack is appropriately chosen depending upon the use or object ofrubber, but is preferably in the range of 10/90 to 99/1, more preferably30/70 to 95/5 and more preferably 50/50 to 90/10, by weight.

When silica is contained as a reinforcing agent in the rubbercomposition of the present invention, a silane coupling agent ispreferably incorporated in addition to silica for further reducing theheat build-up and enhancing the abrasion resistance. The silane couplingagent is not particularly limited, and includes, for example,vinyltriethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,bis[3-(triethoxysilyl)propyl]tetrasulfide,bis[3-(triethoxysilyl)propyl]ditstrasulfide; and tetrasulfides such asγ-trimethoxysilylpropyldimethylthiocarbamyltetrasulfide andγ-trimethoxysilylpropylbenzothiazyltetrasulfide as described in JapaneseUnexamined Patent Publication No. H6-248116. A silane coupling agenthaving not larger than 4 carbon atoms is preferably used becauseundesirable scorch can be avoided at kneading.

The silane coupling agent may be used either alone or in combination.The amount of silane coupling agent is such that the lower limit thereofis preferably 0.1 part by weight, more preferably 1 part by weight andespecially preferably 2 parts by weight, and the upper limit thereof ispreferably 30 parts by weight, more preferably 20 parts by weight andespecially preferably 10 parts by weight, based on 100 parts by weightof silica.

Other Ingredients

The rubber composition of the present invention can contain desiredamounts of various ingredients other than the above-mentionedoil-extended rubber and reinforcing agent, such as a crosslinking agent,a crosslinking accelerator, an accelerator activator, an antioxidant, anactivator, a plasticizer, a lubricant and a filler.

When a rubber ingredient comprising a conjugated diene rubber is used asan oil-extended rubber, an additional rubber can be incorporated inaddition to that contained in the oil-extended rubber, in the rubbercomposition. If the additional rubber is incorporated, the total weightof rubbers in the rubber composition increases, on the basis of whichthe amounts of reinforcing agent and other ingredients are determined.Namely, the amounts of reinforcing agent and other ingredients should bedetermined based on the total weight of the rubber including theadditional rubber.

The content of vinyl bond units in the total conjugated diene rubberconstituting the rubber ingredient in the rubber composition is the sameas that described with regard to the oil-extended rubber of the presentinvention, that is, the lower limit thereof is 20% by weight, preferably40% by weight and more preferably 50% by weight, and the upper limitthereof is preferably 90% by weight, more preferably 85% by weight andespecially preferably 80% by weight, based on the total weight of theconjugated diene monomer units in the conjugated diene rubber.

The content of the conjugated diene rubber in the rubber ingredientcontained in the rubber composition of the present invention is suchthat the lower limit thereof is preferably 50%, more preferably 60% andespecially preferably 70%, and the upper limit is 100%, based on theweight of the rubber ingredient.

When a conjugated diene rubber having a polar group is incorporated, thecontent of conjugated diene rubber having a polar group, especially bothof an amino group and an alkylsilyl group, in the rubber ingredient isalso such that the lower limit thereof is preferably 10% by weight, morepreferably 20% by weight, especially preferably 30% by weight and mostpreferably 40% by weight, and the upper limit thereof is 100% by weight,based on the total weight of the rubber ingredient. When an additionalrubber is used in addition to the oil-extended rubber, a rubbercomposition of the present invention should be formulated so that acomposition satisfying these requirements is obtained.

The total acid value of the process oil contained in the rubbercomposition is such that the upper limit thereof is preferably 1.0mgKOH/g, more preferably 0.4 mgKOH/g especially preferably 0.1 mgKOH/g.The pour point of the process oil is such that the upper limit thereofis 50° C., preferably 30° C. and more preferably 10° C., and the lowerlimit thereof is preferably −20° C. and more preferably −10° C. Thearomatic carbon content (CA%) of the process oil as measured by theKurtz analysis method is such that the lower limit thereof is 20%,preferably 22% and more preferably 25%, and the upper limit thereof is100%.

An additional process oil can be incorporated, in addition to thatcontained in the oil-extended rubber, in the rubber composition of thepresent invention. However, the total of the process oil contained inthe rubber composition having incorporated therein the additionalprocess oil must satisfy the above-mentioned requirements. As theprocess oil to be additionally incorporated, that which satisfies theabove requirements is preferably used.

The paraffinic carbon content (CP%) of the process oil contained in therubber composition is such that the lower limit thereof is 0%, and theupper limit thereof is preferably 55%, more preferably 50% andespecially preferably 45%. The content of a polycyclic aromatic in theprocess oil is preferably below 3%.

The naphthenic carbon., content (CN%) in the process oil is notparticularly limited, which is a value obtained by deducting the sum ofCA% and CP% from 100%.

The amount of the process oil in the rubber composition is such that thelower limit thereof is preferably 5 parts by weight, more preferably 10parts by weight and especially preferably 20 parts by weight, and theupper limit thereof is preferably 100 parts by weight more preferably 80parts by weight and especially preferably 60 parts by weight, based on100 parts by weight of the rubber ingredient. When an additional rubberand/or an additional process oil is incorporated in addition to thoseincorporated as the oil-extended rubber, the resulting rubbercomposition should satisfy this requirement.

A crosslinking agent is further incorporated in the rubber compositionof the present invention, and thus, the rubber composition is used as acrosslinkable rubber composition.

The crosslinking agent is not particularly limited, and includes, forexample, sulfur such as powdery sulfur, precipitated sulfur, colloidalsulfur, insoluble sulfur and highly dispersible sulfur; halogenatedsulfur such as sulfur monochloride and sulfur dichloride; organicperoxides such as dicumyl peroxide and di-tert-butyl peroxide, quinonedioximes such as p-quinone dioxime and p,p′-dibenzoylquinone dioxime;organic polyamines such as triethylenetetraamine, hexamethylenediaminecarbamate and 4,4′-methylene-bis-o-chloroaniline; and alkyl phenolresins having a methylol group. Of these, sulfur is preferable. Powderysulfur is especially preferable. These crosslinking agents may be usedeither alone or as a combination of at least two thereof.

The amount of crosslinking agent is such that the lower limit thereof ispreferably 0.1 part by weight, more preferably 0.3 part by weight andespecially preferably 0.5 part by weight, and the upper limit thereof ispreferably 15 parts by weight, more preferably 10 parts by weight andespecially preferably 5 parts by weight, based on 100 parts by weight ofthe rubber ingredient. When the amount of crosslinking agent is withinthis range, a crosslinked rubber product made from the rubbercomposition has greatly reduced heat build-up, and greatly improvedtensile strength and abrasion resistance.

As specific examples of the crosslinking accelerator, there can bementioned sulfenamide crosslinking accelerators such asN-cyclohexyl-2-benzothazole sulfenamide, N-t-butyl-2-benzothazolesulfenamide, N-oxyethylene-2-benzothazole sulfenamide andN,N′-diidsopropyl-2-benzothazole sulfenamide; guanidine crosslinkingaccelerators such as diphenylguanidine, diorthotolylguanidine andorthotolylbiguanidine; thiourea crosslinking accelerators such asdiethylthiourea; thiazole crosslinking accelerators such as2-mercaptobenzothiazole, dibenzothiazyl disulfide and2-mercaptobenzothiazole zinc salt; thiuram crosslinking acceleratorssuch as tetramethylthiuram monosulfide and tetramethylthiuram disulfide;dithiocarbamate crosslinking accelerators such as sodiumdimethyldithiocarbamate and zinc dimethyldithiocarbamate; andxanthogenate crosslinking accelerators such as sodiumisopropylxanthogenate, zinc isopropylxanthogenate and zincbutylxanthogenate.

These crosslinking accelerators may be used either alone or as acombination of at least two thereof. Sulfenamide crosslinkingaccelerators are preferable. The amount of crosslinking accelerator issuch that the lower limit thereof is preferably 0.1 part by weight, morepreferably 0.3 part by weight and especially preferably 0.5 part byweight, and the upper limit thereof is preferably 15 parts by weight,more preferably 10 parts by weight and especially preferably 5 parts byweight, based on 100 parts by weight of the rubber ingredient.

The accelerator activator is not particularly limited, and includes, forexample, higher fatty acids such as stearic acid, and zinc oxide. Aszinc oxide, those which have a particle diameter of not larger than 5 μmand thus have a high surface activity are preferable. As specificexamples of the preferable zinc oxide, there can be mentioned activezinc oxide having a particle diameter in the range of 0.05 to 0.2 μm andzinc oxide having a particle diameter in the range of 0.3 to 1 μm. Thezinc oxide can be surface-treated with an amine dispersing agent or awetting agent.

The accelerator activator may be used either alone or as a combinationof at least two thereof. The amount of accelerator activator isappropriately chosen depending upon the particular kind of acceleratoractivator. The amount of a higher fatty acid is such that the lowerlimit thereof is preferably 0.05 part by weight, more preferably 0.1part by weight and especially preferably 0.5 part by weight, and theupper limit thereof is preferably 15 parts by weight, more preferably 10parts by weight and especially preferably 5 parts by weight, based on100 parts by weight of the rubber ingredient. The amount of zinc oxideis such that the lower limit thereof is preferably 0.05 part by weight,more preferably 0.1 part by weight and especially preferably 0.5 part byweight, and the upper limit thereof is preferably 10 parts by weight,more preferably 5 parts by weight and especially preferably 2 parts byweight, based on 100 parts by weight of the rubber ingredient. When theamount of accelerator activator is within this range, a crossslinkedrubber product made from the rubber composition exhibits good andbalanced processability, tensile strength and abrasion resistance.

Other ingredients, for example, activators such as diethylene glycol,polyethylene glycol and silicone oil, fillers such as calcium carbonate,talc and clay; and wax can be incorporated in the rubber composition.

The rubber composition of the present invention can be prepared bykneading the respective ingredients by a conventional procedure. Forexample, a rubber ingredient is mixed with ingredients other than acrosslinking agent and a crosslinking accelerator, and then, the rubberingredient-mixture is mixed with a crosslinking agent and a crosslinkingaccelerator to give the rubber composition. The temperature at which arubber ingredient is mixed with ingredients other than a crosslinkingagent is such that the lower limit thereof is preferably 80° C., morepreferably 100° C. and especially preferably 140° C., and the upperlimit thereof is preferably 200° C., more preferably 190° C. andespecially preferably 180° C. The time for which a rubber ingredient ismixed with ingredients other than a crosslinking agent is such that thelower limit thereof is preferably 30 seconds and more preferably 1minute, and the upper limit thereof is preferably 30 minutes. Mixing ofa crosslinking agent and a crosslinking accelerator with therubber-containing mixture is carried out after the rubber-containingmixture is cooled usually to a temperature not higher than 100° C.,preferably not higher than 80° C.

Cross-Linked Article

The rubber composition of the present invention is generally used as acrosslinked rubber article made therefrom. The rubber composition usedhas incorporated therein a crosslinking agent, a crosslinkingaccelerator and other ingredients required for crosslinking.

The procedure for crosslinking is not particularly limited, and may bechosen depending upon the particular shape and size of a crosslinkedrubber article. A crosslinkable rubber composition can be filled andheated in a mold whereby crosslinking is effected simultaneously withshaping into a crosslinked rubber article. Alternatively, acrosslinkable rubber composition can be preliminarily shaped into arubber article, and then the article is heated to be therebycrosslinked.

The crosslinking temperature and tine are also not particularly limited,and may be chosen depending upon the particular shape and size of acrosslinked rubber article. The crosslinking temperature is generallysuch that the lower limit thereof is preferably 120° C., more preferably140° C., and the upper limit thereof is preferably 200° C., morepreferably 180° C.

The invention will now be specifically described by the followingproduction examples, examples and comparative examples. In theseexamples, parts are by weight. The properties of polymers, process oiland rubbers are evaluated by the following methods.

(1) Total Acid Value of Process Oil

The total acid value (mgKOH/g) of process oil was measured by apotentiometric titration method according to JIS K2501.

(2) Bound Styrene Content in Polymer

The content (%) of bound styrene in a polymer was determined by arefractive index method according to K6383.

(3) Vinyl Bond Unit Content in Conjugated Diene Units

The content (%) of vinyl bond units in conjugated diene units of apolymer bound was determined by infrared spectrophotometry (Hamptonmethod).

(4) Molecular Weight of Polymer

The weight average molecular weight (Mw) and number average molecularweight (Mn) of a polymer were measured by GPC and expressed in terms ofthose of polystyrene. The weight average molecular weight (Mw) andmolecular weight distribution (Mw/Mn) of a polymer were determined.

(5) Sequence Distribution of Styrene Units

The sequence distribution of styrene units was determined by the methoddescribed in Preprint of Polymer Society of Japan, vol. 29, No. 9,P2055-. Namely, a copolymer was subjected to ozonolysis and the productwas measured by GPC. The content (%) of an isolated short chaincomprised of one styrene unit, and the content (%) of a long sequencecomposed of at least eight styrene units, in the total styrene units,were calculated.

(6) Tensile Strength

The tensile strength of rubber was expressed by the modulus in tensionat an elongation of 300% (kgf/cm²). The 300% modulus in tension wasmeasured according to JIS K6301, and was expressed by an index number(tensile strength index number). The larger the index number, the morepreferable the rubber.

(7) Heat Build-Up

Tan δ was measured at a twist of 0.5%, a frequency of 20 Hz, and atemperatures of 0° C. and 60° C. by using “RDA-II” available fromRheometrios Co. The heat build-up-was expressed by an index number (heatbuild-up index number) of the ratio of tan δ (0° C.)/tan δ (60° C.) asthe tan δ ratio of a comparative example is 100. The larger the heatbuild-up, index number, the more preferable the heat build-up of rubber.

(8) Abrasion Resistance

The abrasion resistance of rubber was measured according to ASTM D2228by using a Pico abrasion machine. The abrasion resistance was expressedby an index number (abrasion resistance index number). The larger theabrasion resistance index number, the more preferable the abrasionresistance of rubber.

PRODUCTION EXAMPLE 1 Production of Conjugated Diene Rubber 1 andOil-Extended Rubber 1

An autoclave equipped with a stirrer was charged with 4,000 g ofcyclohexane, 140 g of styrene, 460 g of 1,3-butadiene and 12 milli-moleof tetramethylethylenediamine, followed by addition of 4.66 milli-moleof n-butyllithium to initiate polymerization at 40° C. When 20 minuteselapsed from the commencement of polymerization, a mixture of 70 g ofstyrene and 330 g of 1,3-butadiene was continuously added. After it waschecked that the conversion reached 100%, 0.42 milli-mole oftetramethoxysilane was added and a reaction was conducted for 30minutes. The highest temperature of a polymerization mixture was 60° C.After completion of the reaction, 10 milli-mole of methanol as aterminator was added to give a polymer solution.

A part of the polymer solution was collected, and then dropped intomethanol to precipitate a rubber ingredient. The rubber ingredient wasdried by a vacuum drier. Thus, a small amount of conjugated diene rubber(rubber 1) was obtained. Molecular weight and other properties of rubber1 are shown in Table 1.

To the above-mentioned polymer solution, 0.2 part of2,4-bis(n-octylthiomethyl)-6-methylphenol and 37.5 parts of process oil(oil 1 shown in Table 2), each per 100 parts of rubber 1 in the polymersolution, were added. A polymer was recovered by a steam-strippingmethod, dehydrated by rolls and then dried by a hot air dryer to giveoil-extended rubber 1. Oil-extended rubber 1 had a Mooney viscosity of55. At the stripping step, polyoxyethylene polyoxypropylene ether wasadded to water in a stripping zone in an amount such that theconcentration thereof was 20 ppm, and the concentration of a crumb-formoil-extended rubber was 5% by weight based on the weight of water in thestripping zone.

PRODUCTION EXAMPLE 2 to 5 Production of Oil-Extended Rubbers 2 to 5

By the same procedures as described in Production Example 1, rubber 1was extended with process oil wherein oil 2, oil 3, oil 4 and oil 5 wereused in Production Examples 2, 3, 4 and 5, respectively, to obtainoil-extended rubbers 2, 3, 4 and 5, with all other conditions remainingthe same. The Mooney viscosities of oil-extended rubbers 2, 3, 4 and 5were 56, 51, 53 and 45, respectively.

PRODUCTION EXAMPLE 3 TO 5 Production of Conjugated Diene Rubber 2 andOil-Extended Rubbers 6 and 7

An autoclave equipped with a stirrer was charged with 4,000 g ofcyclohexane, 160 g of styrene, 440 g of 1,3-butadiene and 12 milli-moleof tetramethylethylenediamine, followed by addition of 5.00 milli-moleof n-butyllithium to initiate polymerization at 40° C. When 15 minuteselapsed from the commencement of polymerization, a mixture of 90 g ofstyrene and 310 g of 1,3-butadiene was continuously added. After it waschecked that the conversion reached 100%, 0.74 milli-mole oftetramethoxysilane was added and a reaction was conducted for 30minutes. The highest temperature of a polymerization mixture was 60° C.After completion of the reaction, 10 milli-mole of methanol as aterminator was added to give a polymer solution.

A part of the polymer solution was collected, and then dropped intomethanol to precipitate a rubber ingredient. The rubber ingredient wasdried by a vacuum drier. Thus, a small amount of a conjugated dienerubber (rubber 2) was obtained. Molecular weight and other properties ofrubber 2 are shown in Table 1.

By the same procedures as described in Production Example 1, rubber 2 inplace of rubber 1 was extended with process oil wherein oil 1 and oil 3were used in Production Examples 6 and 7, respectively, to obtainoil-extended rubbers 6 and 7, with all other conditions remaining thesame. The Mooney viscosities of oil-extended rubbers 6 and 7 were 45 and44, respectively.

PRODUCTION EXAMPLE 8 Production of Conjugated Diene Rubber 3 andOil-Extended Rubber 8

Two autoclaves (first and second autoclaves) each equipped with astirrer were connected in series. A mixture of styrene and 1,3-butadiene(weight ratio 25/75), 400 g of cyclohexane per 100 g of thestyrene/1,3-butadiene mixture, and 0.17 g of tetramethylethylenediamine,0.034 g of n-butyllithium, 0.43 g of 1,2-butadiene and 0.9 g ofdivinylbenzene, each per 100 g of the styrene/1,3-butadiene mixture,were continuously fed into the first autoclave to effect polymerization.The feed rate was controlled so that the average residence time in eachautoclave was 2 hours. The first and second autoclaves were maintainedat a temperature of 55° C. and 80° C., respectively. After completion ofthe reaction, 0.034 g of methanol, per long of the styrene/1,3-butadienemixture, as a terminator was added to give a polymer solution. By thesame procedures as described in Production Example 1, a small amount ofconjugated diene rubber (rubber 3) was obtained. Molecular weight andother properties of rubber 3 are shown in Table 1.

To the above-mentioned polymer solution, 0.1 part of2,6-di-t-butylphenol and 37.5 parts of process oil (oil 4 shown in Table2), each per 100 parts of rubber 3 in the polymer solution, were added.A polymer was recovered by a steam-stripping method in the same manneras in Production Example 1, dehydrated by rolls and then dried by a hotair dryer to give oil-extended rubber 8. Oil-extended rubber 8 had aMooney viscosity of 50.

PRODUCTION EXAMPLE 9 Production of Conjugated Diene Rubber 4 andOil-Extended Rubber 9

By the same procedures as described in Production Example 8, conjugateddiene rubber 4 and oil-extended rubber 9 were produced whereindivinylbenzene was not added and 0.20 g of tetramethoxysilane, per 100 gof the styrene/1,3-butadiene mixture, was continuously fed to the secondautoclave, and oil 6 shown in Table 2 was used as process oil, with allother conditions remaining the same. Molecular weight and otherproperties of rubber 4 are shown in Table 1. Oil-extended rubber 9 had aMooney viscosity of 60.

PRODUCTION EXAMPLE 10 Production of Conjugated Diene Rubber 5 andOil-Extended Rubber 10

An autoclave equipped with a stirrer was charged with 4,000 g ofcyclohexane, 150 g of styrene, 450 g of 1,3-butadiene and 10 milli-moleof tetramethylethylenediamine, followed by addition of 6.3 milli-mole ofn-butyllithium to initiate polymerization at 50° C. When 10 minuteselapsed from the commencement of polymerization, a mixture of 50 g ofstyrene and 350 g of 1,3-butadiene was continuously added. After it waschecked that the conversion reached 100%, 1.1 milli-mole oftetramethoxysilane was added and a reaction was conducted for 30minutes. The highest temperature of a polymerization mixture was 80° C.After completion of the reaction, 10 milli-mole of methanol as aterminator was added to give a polymer solution. A small amount ofrubber 5 was obtained from the thus-produced polymer solution. Molecularweight and other properties of rubber 5 are shown in Table 1.

To the above-mentioned polymer solution, 0.1 part of2,6-di-t-butylphenol and 37.5 parts of process oil (oil 5 shown in Table2), each per 100 parts of rubber 5 in the polymer solution, were added.A polymer was recovered by a steam-stripping method in the same manneras in Production Example 1, dehydrated by rolls and then dried by a hotair dryer to give oil-extended rubber 10. Oil-extended rubber 10 had aMooney viscosity of 37.

PRODUCTION EXAMPLE 11 Production of Conjugated Diene Rubber 6 andOil-Extended Rubber 11

An autoclave equipped with a stirrer was charged with 4,000 g ofcyclohexane, 150 g of styrene, 720 g of 1,3-butadiene and 2.6 milli-moleof tetramethylethylenediamine, followed by addition of 7.6 milli-mole ofn-butyllithium to initiate polymerization at 40° C. When 40 minuteselapsed from the commencement of polymerization, 130 g of 1,3-butadienewas continuously added. After it was checked that the conversion reached100%, 5.3 milli-mole of tin tetrachloride was added and a reaction wasconducted for 5 minutes. Further, 4.8 milli-mole ofN,N′-diemthylethylene urea was added and a reaction was conducted for 20minutes. After completion of the reaction, 20 milli-mole of methanol asa terminator was added to give a polymer solution. A small amount ofrubber 6 was obtained from the thus-produced polymer solution. Molecularweight and other properties of rubber 6 are shown in Table 1.

To the above-mentioned polymer solution, 0.1 part of2,6-di-t-butylphenol and 37.5 parts of process oil (oil 5 shown in Table2), each per 100 parts of rubber 6 in the polymer solution, were added.A polymer was recovered by a steam-stripping method in the same manneras in Production Example 1, dehydrated by rolls and then dried by a hotair dryer to give oil-extended rubber 11. Oil-extended rubber 11 had aMooney viscosity of 12.

PRODUCTION EXAMPLE 12 Production of Conjugated Diene Rubber 7 andOil-Extended Rubber 12

By the same procedures as described in Production Example 2, rubber 7was produced wherein tin tetrachloride was used instead oftetramethoxysilane with all other conditions remaining the same.Molecular weight and other properties of rubber 7 are shown in Table 1.

To the thus-obtained polymer solution, 0.2 part of2,4-bis(n-octylthiomethyl)-6-methylphenol was added per 100 parts ofrubber 7 in the polymer solution. To 100 parts of oil 3, 4 milli-mole oflithium hydroxide was added and the mixture was stirred at 50° C. for 24hours to give oil 7. 37.5 parts of oil 7 as process oil, per 100 partsof rubber 7 in the polymer solution, was added to the polymer solution.A polymer was recovered by a steam-stripping method in the same manneras in Production Example 1, dehydrated by rolls and then dried by a hotair dryer to give oil-extended rubber 12. Oil-extended rubber 12 had aMooney viscosity of 44. Total acid value and other properties of oil 7are shown in Table 2.

PRODUCTION EXAMPLE 13 Production of Conjugated Diene Rubber 8 andOil-Extended Rubber 13

By the same procedures as described in Production Example 10, rubber 8was produced wherein tin tetrachloride was used instead oftetramethoxysilane with all other conditions remaining the same.Molecular weight and other properties of rubber 7 are shown in Table 1.By the same procedures as described in Production Example 10,oil-extended rubber 13 was produced wherein oil 4 was used as processoil with all other conditions remaining the same. Oil-extended rubber 13had a Mooney viscosity of 32.

TABLE 1 Rubber No. 1 2 3 4 5 6 7 8 Content of bound 21.2 25.0 25.4 32.319.8 15.0 24.9 20.2 styrene (%) Content of vinyl 70.2 70.1 69.9 36.660.7 42.4 70.2 59.8 bond units (%)*1 Content of isolated 87 88 61 58 8869 86 87 styrene unit (%)*2 Content of long styrene 0 0 1.4 1.8 0 0.3 00 sequence (%)*3 Mw/10,000 80 75 66 70 72 40 78 71 Mw/Mn 1.7 1.8 2.4 3.21.7 1.6 1.8 1.8 Coupling percent (%) 43 50 0 0 58 30 52 54 *1Content ofvinyl bond units in the conjugated diene units (%) *2Content of isolatedshort chain of styrene unit (%) *3Content of long sequence of styreneunits (%)

Rubbers 1, 2 and 5 to 8 have a vinyl bond unit content of at least 20%and a molecular weight distribution (Mw/Mn) of not larger 2.2, and thus,these rubbers can be used in the present invention as they are. Rubbers3 and 4 have a vinyl bond unit content of at least 20%, but have amolecular weight distribution (Mw/Mn) of larger 2.2.

TABLE 2 Oil No. 1 2 3 4 5 6 7 Total acid value 0.3 0.3 0.2 5.4 0.01 0.20 Pour point 5 −2.5 27 −2.5 −18 18 27 CA % 45 44 27 39 6 44 27 CN % 2732 32 32 37 25 32 CP % 28 24 41 29 58 31 41 Polycyclic structure 5.7 6.62.1 11.8 1.0 6.9 2.2 content

Oils 1 to 3, 6 and 7 have a total acid value of not larger than 1.0mgKOH/g, a pour point of not higher than 50° C. and CA% of at least 20%,and thus, are used in the present invention. Oil 5 has a small CA%,i.e., 6%. Oil 4 has a large total acid value, i.e., that is larger than1.0 mgKOH/g.

EXAMPLES 1 TO 3, COMPARATIVE EXAMPLES 6and 7

Using each of oil-extended rubbers 1 to 5 as a raw material rubber, arubber composition was prepared. That is, 137.5 parts of a raw materialrubber (comprised of 100 parts of rubber ingredient and 37.5 parts ofprocess oil), 80 parts of carbon black N220 (“Seast 6”® available fromTokai Carbon K.K., nitrogen adsorption specific-surface area: 119 m²/g,DBP adsorption specific surface area 114 ml/100 g), 3 parts of zincoxide, 2 parts of stearic acid and 2 parts of antioxidant[N-(1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine)] were kneadedtogether at 140° C. for 4 minutes by a 250 ml Brabender mixer.

Then the kneaded mixture was kneaded together with 1.4 parts of sulfurand 1.6 parts of crosslinking accelerator(N-cyclohexyl-2-benzothiazolesulfenamide) at 50° C. by open rolls toprepare rubber compositions 1 to 5.

Each of the thus-prepared rubber compositions 1 to 5 was press-cured at160° C. for 15 minutes to prepare a sheet with a thickness of 2 mm and atest specimen for Pico abrasion test. Properties of the thus-crosslinkedrubber were evaluated. The evaluation results are expressed in Table 3by an index number as the results in Comparative Example 1 are 100.

TABLE 3 Examples Comp. Ex. 1 2 3 1 2 Oil-unextended rubber No. 1 1 1 1 1Process oil No. 1 2 3 4 5 Oil-extended rubber No. 1 2 3 4 5 Rubbercomposition No. 1 2 3 4 5 Tensile strength index 110 109 113 100 120Heat build-up index 102 103 101 100 100 Abrasion resistance index 108108 104 100 88

EXAMPLES 4 and 5, COMPARATIVE EXAMPLES 3 and 6

Using each of oil-extended rubbers 6 to 11 as a raw material rubber, arubber composition was prepared. That is, 137.5 parts of a raw materialrubber (comprised of 100 parts of rubber ingredient and 37.5 parts ofprocess oil), 40 parts of silica (“Zeosil”® 1165MP available from RhodiaCo.), and 3.2 parts of a silane coupling agent (Si69 available fromDegussa Huls AG.) were kneaded together at 150° C. for 2 minutes by a250 ml Brabender mixer. Then, to the kneaded mixture, 40 parts of silica(“Zeosil”® 1165MP available from Rhodia Co.), 3.2 parts of a silanecoupling agent (Si69 available from Degussa Co.), 1.5 parts of zincoxide, 2 parts of stearic acid and 2 parts of an antioxidant (“Nocrac”®6C, available from Ouchi Shinko K.K.,N-(1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine)) were added and themixture was further kneaded at 150° C. for 3 minutes. Then, to thethus-obtained mixture, 1.4 parts of sulfur, and 3.5 parts of acrosslinking accelerator, (a mixture of 1.7 parts ofN-cyclohexyl-2-benzothiazolesulfenamide and 1.8 parts ofdiphenylguanidine), were added and kneaded together at 50° C. by openrolls to prepare rubber compositions 6 to 11.

Each of the thus-prepared rubber compositions 6 to 11 was press-cured at160° C. for 15 minutes to prepare a sheet with a thickness of 2 mm and atest specimen for Pico abrasion test. Properties of the thus-crosslinkedrubber were evaluated. The evaluation results are expressed in Table 4by an index number as the results in Comparative Example 3 are 100.

TABLE 4 Examples Comparative Examples 4 5 3 4 5 6 Oil-unextended rubberNo. 2 2 3 4 5 6 Process oil No. 1 3 4 6 5 5 Oil-extended rubber No. 6 78 9 10 11 Rubber composition No. 6 7 8 9 10 11 Tensile strength index115 119 100 98 110 104 Heat build-up index 114 117 100 92 93 68 Abrasionresistance index 110 105 100 111 96 91

EXAMPLES 6 to 8, COMPARATIVE EXAMPLES 7 to 10

Using each of oil-extended rubbers 6 to 10, 12 and 13 as a raw materialrubber, a rubber composition was prepared. That is, 110 parts of a rawmaterial rubber, 20 parts of butadiene rubber (“Nipol”® BR-1220,available from Nippon Zeon Co., Mooney viscosity: 43, cis-bond content:at lesat 98%), 40 parts of silica shown in Table 5, and 3.2 parts of asilane coupling agent (Si69 available from Degussa Huls AG.) werekneaded together at 150° C. for 2 minutes by a 250 ml Brabender mixer.

Then, to the kneaded mixture, 40 parts of silica shown in Table 5, 10parts of carbon black N220, 10 parts of process oil shown in Table 5 asincorporated oil, 2 parts of a silane coupling agent (Si69 availablefrom Degussa Co.), 1.5 parts of zinc oxide, 2 parts of stearic acid and2 parts of an antioxidant (“Nocrac”® 6C, available from Ouchi ShinkoK.K.) were added and the mixture was further kneaded at 150° C. for 3minutes.

Then, to the thus-obtained mixture, 1.4 parts of sulfur, and 3.2 partsof a crosslinking accelerator, (a mixture of 1.7 parts of “Nocceler”®CZ, available-from Ouchi Shinko K.K.,N-cyclohexyl-2-benzothiazylsulfenamide and 1.5 parts of “Noccceler”® D,available from Ouchi Shinko K.K., diphenylguanidine), were added andkneaded together at 50° C. by open rolls to prepare rubber compositions12 to 18.

Each of the thus-prepared rubber compositions 12 to 18 was press-curedat 160° C. for 15 minutes to prepare a sheet with a thickness of 2 mmand a test specimen for Pico abrasion test. Properties of thethus-crosslinked rubber were evaluated. The evaluation results areexpressed in Table 5 by an index number as the results in ComparativeExample 7 are 100.

TABLE 5 Examples Comparative Examples 4 5 3 4 5 6 Oil-unextended rubber2 2 7 3 4 5 8 No. Process oil No. 1 3 7 4 6 5 4 Oil-extended rubber 6 712 8 9 10 13 No. Rubber composition 12 13 14 15 16 17 18 No. Silica MPMP MP MP VN VN VN Incorporated oil No. 1 3 7 4 5 6 4 Tensile strengthindex 111 115 115 100 93 102 95 Heat build-up index 118 126 134 100 7378 58 Abrasion resistance 113 107 109 100 102 89 100 index Abbreviationsin Table 5 are as follows. VN “Npsil” ™ VN3, available from NipponSilica K.K., nitrogen adsorption specific surface area 240 m²/g MP“Zeosil” ™ 1165MP, available from Rhodia Co., nitrogen adsorptionspecific surface area 175 m²/g

Among the rubbers used in the Comparative Examples, rubber 3 and rubber4 have a too large molecular weight distribution and cannot be usedalone in the present invention.

In Comparative Examples 3 and 7, only rubber 3 was used as the rubberingredient. In Comparative Examples 4 and 8, rubber 4 was used as therubber ingredient. These rubbers do not contain a conjugated dienerubber having a molecular weight distribution (Mw/Mn) of 1.2 to 2.2. InComparative Examples 1, 3, 7 and 10, oil 4 was used as process oil,which had a too large total acid value as compared with that of processoil used in the present invention. In Comparative Examples 2, 5, 6 and9, oil 5 was used as process oil, which had a too small aromatic carboncontent as compared with that of process oil used in the presentinvention.

As seen from comparison of the examples with comparative examples,crosslinked rubbers of the examples exhibited tensile strength index,heat build-up index and abrasion resistance index, all of which werelarger than the standard value in comparative examples. In contrast,crosslinked rubbers of the comparative examples exhibited thesecharacteristics, at least one of which was smaller than the standardvalue in comparative examples. It is to be noted that asilica-incorporated rubber composition exhibited excellent heatbuild-up.

INDUSTRIAL APPLICABILITY

The rubber composition of the present invention gives a crosslinkedrubber product having high tensile strength, high abrasion resistanceand reduced heat build-up. Therefore, the rubber composition is suitablefor automobile tire material. Especially silica incorporated rubbercomposition is useful for an automobile tire having a reduced fuelconsumption.

What is claimed is:
 1. An oil-extended rubber comprising 100 parts byweight of a rubber ingredient comprising a conjugated diene rubberwherein the ratio (Mw/Mn) of weight average molecular weight (Mw) tonumber average molecular weight (Mn), as measured by gel permeationchromatography and expressed in terms of those of polystyrene, is in therange of 1.2 to 2.2, and the conjugated diene monomer units have a vinylbond unit content of at least 20%, and 5 to 100 parts by weight of aprocess oil having a total acid value of not larger than 1.0 mgKOH/g, apour point of not higher than 50° C. and an aromatic carbon content ofat least 20% as measured by the Kurtz analysis method.
 2. Anoil-extended rubber according to claim 1, wherein the content of theconjugated diene rubber is in the range of 50 to 100% by weight based onthe weight of the rubber ingredient.
 3. An oil-extended rubber accordingto claim 1, wherein the conjugated diene rubber is a polymer of aconjugated diene monomer or monomers, or a copolymer of a conjugateddiene monomer with a monomer copolymerizable with the conjugated dienemonomer.
 4. An oil-extended rubber according to claim 3, wherein theconjugated diene rubber is a copolymer of a conjugated diene monomerwith a monomer copolymerizable therewith, wherein the content of anisolated short chain of a copolymerizable monomer unit is in the rangeof 50 to 100% by weight.
 5. An oil-extended rubber according to claim 3,wherein the conjugated diene rubber is a copolymer of a conjugated dienemonomer with a monomer copolymerizable therewith, wherein the content ofa long sequence of copolymerizable monomer units is in the range of 0 to2% by weight.
 6. An oil-extended rubber according to claim 1, whereinthe conjugated diene rubber has a weight average molecular weight (Mw)of 200,000 to 2,000,000.
 7. An oil-extended rubber according to claim 1,wherein the amount of the process oil is in the range of 5 to 60 partsby weight.
 8. An oil-extended rubber according to claim 1, wherein theoil-extended rubber has a Mooney viscosity of 20 to
 100. 9. A processfor producing an oil-extended rubber as claimed in claim 1, whichcomprises the steps of: (1) polymerizing a conjugated diene monomer ormonomers alone, or a combination of a conjugated diene monomer ormonomers with a monomer copolymerizable therewith by using an organicactive metal initiator in the presence of a polar compound in ahydrocarbon medium, (2) adding a terminator to a polymerization systemto terminate the polymerization, (3) adding 5 to 100 parts by weight,based on 100 parts by weight of the total monomers, of a process oilhaving a total acid value of not larger than 1.0 mgKOH/g, a pour pointof not higher than 50° C. and an aromatic carbon content of at least 20%as measured by the Kurtz analysis method, and (4) removing thehydrocarbon medium and recovering the thus-produced oil-extended rubber.10. A rubber composition comprising (i) 100 parts by weight of a rubberingredient comprising a conjugated diene rubber wherein the ratio(Mw/Mn) of weight average molecular weight to number-average molecularweight (Mn), as measured by gel permeation chromatography and expressedin terms of those of polystyrene, is in the range of 1.2 to 2.2, and theconjugated diene monomer units have a vinyl bond unit content of atleast 20%, (ii) 5 to 100 parts by weight of a process oil having a totalacid value of not larger than 1.0 mgKOH/g, a pour point of not higherthan 50° C. and an aromatic carbon content of at least 20% as measuredby the Kurtz analysis method, and (iii) 10 to 200 parts by weight of areinforcing agent.
 11. A rubber composition according to claim 10,wherein the amount of the conjugated diene rubber is in the range of 50to 100% by weight based on the weight of the rubber ingredient.
 12. Arubber composition according to claim 10, wherein the conjugated dienerubber is a polymer of a conjugated diene monomer or monomers, or acopolymer of a conjugated diene monomer with a monomer copolymerizablewith the conjugated diene monomer.
 13. A rubber composition according toclaim 12, wherein the conjugated diene rubber is a copolymer of aconjugated diene monomer with a monomer copolymerizable therewith,wherein the content of an isolated short chain of a copolymerizablemonomer unit is in the range of 50 to 100% by weight.
 14. A rubbercomposition according to claim 12, wherein the conjugated diene rubberis a copolymer of a conjugated diene monomer with a monomercopolymnerizable therewith, wherein the content of a long sequence ofcopolymnerizable monomer units is in the range of 0 to 2% by weight. 15.A rubber composition according to claim 11, wherein the conjugated dienerubber has a weight average molecular weight (Mw) of 200,000 to2,000,000.
 16. A rubber composition according to claim 10, wherein thereinforcing agent is at least one member selected from the groupconsisting of silica and carbon black.
 17. A rubber compositionaccording to claim 16, wherein the silica has a nitrogen absorptionspecific surface area of 50 to 400 m²/g as measured by the BET method.18. A rubber composition according to claim 16, wherein the carbon blackhas a nitrogen absorption specific surface area of 5 to 200 m²/g asmeasured by the BET method.
 19. A rubber composition according to claim16, wherein the carbon black has a dibutyl phthalate absorption of 5 to300 ml/100 g.
 20. A rubber composition according to claim 16, whereinthe reinforcing agent comprises both of-silica and carbon black, theratio of silica/carbon black being in the range of 10/90 to 99/1 byweight.
 21. A crosslinked rubber article made by crosslinking a rubbercomposition as claimed in claim 10.